Hydrostatically steered tractors have been available for many years and their application to windrowers has been significant. There has been a limit to the transport speed of these windrowers due to instability at high speed with the drive wheels ahead of the castor wheels (cab forward). Higher speeds on the road allow the user to cover large distances without the use of trailers and tow vehicles.
In U.S. Pat. No. 7,306,062 issued Dec. 11, 2007 by inventor Dunn and assigned to the present assignee is shown a tractor of this type in which the machine travels in transport mode with the castor wheels leading the drive wheels (engine forward), and in this position the machine is much more stable and higher speeds are possible.
While in a turn with a self-propelled machine that has the steered wheels leading, the centrifugal force at the center of gravity is such that it tends to assist the rear driving wheels in straightening out the machine. On the other hand when in a turn with a self-propelled machine that has the driving wheels leading, the centrifugal force at the center of gravity is such that it encourages the turn and the front driving wheel must overcome this force in order to straighten out the machine. Thus the machine is more stable, or wants to travel more in a straight line, when the castor wheels are leading and the driven wheel are trailing.
In U.S. Pat. No. 7,159,687 issued Jan. 9, 2007 by inventor Dunn and assigned to the present assignee is shown a typical machine of this type, where the ground speed and steering of the machine is achieved by varying the flow output from two individual hydraulic pumps each driving a hydraulic motor attached to the drive wheels. In an application using hydraulic piston pumps, this flow is varied by changing the angle of the cam-plate or swash plate inside the pump which varies the displacement of the pump.
In US published application 2009/0242219 (Dunn et al) published Oct. 1, 2009 and assigned to the present assignees shows further details of the steering system for a tractor of this general type, the disclosure of which is incorporated therein by reference.
To vary the speed of the machine, both pump displacements are changed simultaneously. To steer the machine, the displacement of each pump is varied relative to each other so that the flow to each wheel is different which causes the machine to turn.
The above patents therefore provide a tractor which can travel at increased speeds both in field operation and also in transport. While relatively low speed movement allows an arrangement in which the cab is mounted directly to the frame. However increased speeds require the addition of an effective suspension system for the tractor to increase the isolation of the operator from rough terrain during field operation.